Asymmetric membranes for use in a variety of purification operations have long been known. Permeation selective or permselective membranes preferentially pass certain components of liquid mixtures while retaining other components. In reverse osmosis membranes, a hydrostatic pressure in excess of the equilibrium osmotic pressure of the liquid mixture or solution is applied to the liquid to force the more permeable components, usually water, through the membrane in preference to the less permeable components, such as a salt, contrary to the normal osmotic flow. In recent years, substantial effort has been directed at desalination of brackish water and sea water on a practical scale. It has been difficult to find permselective barriers that simultaneously exhibit high permeability to water (high water flux), low permeability to dissolved ions (low salt passage), high mechanical strength in order to survive high operating pressures, commonly of the order of 70 kg/cm.sup.2 (1000 psi, and long-term stability in use. Cellulose acetate (CA), much used for permselective barriers, exhibits short life, and decay of performance during use attributed to collapse of the porous membrane at the pressures required which results in decrease of water flux. Aliphatic polyamides are more durable, but exhibit poorer permeation properties and salt rejections; hydrophilically substituted such polyamides have water permeability near that of CA, but inferior physical strength.
Aromatic polyamides suitable for such membranes, and membranes thereof, have been described in U.S. Pat. No. 3,567,632 and in German Published Application No. 27 29 847. Membranes of such polymers are more durable than CA and aliphatic polyamides, and have better permeation properties. However, the use of pendant ionic groups in these polymers to raise the hydrophilicity of the polymer and, thus, the water flux also had three negative effects: (1) increased salt passage, (2) decreased hydrolytic stability, and (3) decreased modulus which results in lower collapse pressure for hollow fiber membranes.
Accordingly, it is a principal object of this invention to provide improved membranes for permeation processes, especially reverse osmosis membranes for purification of water.
It is another object to provide novel aromatic polyamides suitable for, and especially adapted for, use in membranes for such permeation processes.
Other objects will appear hereinafter.